There are many industrial processes, e.g., in the operation of steel-making furnaces, coal-fired electrical power generation plants, and the like, which produce substantial gas flows with particulate matter entrained therein. For such an operation of any significant size the amount of particulate matter or dust entrained in such gas flows is substantial and it is generally illegal, expensive and impractical to release the dust laden gases directly into the atmosphere. In a preliminary step, centrifugal separators, cyclone separators, and the like, can be used to remove the larger particles from the flow. However, the remaining finer particulate matter upon release into the atmosphere tends to disperse widely and therefore tends to irritate even distant neighbors of such an industrial operation and must be filtered out carefully and efficiently.
Simple fabric type filters, generally disposed in plural arrays in so called "baghouses", have been used in the past. Such systems generally require substantial electrical power to operate the blowers that drive the dust-laden gas flow through the filters which tend to get easily clogged and are generally short lived. Numerous solutions have been tried to utilize electrostatic forces to enhance dust filtration in a manner that relieves some of these problems. Each industrial application poses its own unique problems which depend, for example, on the ratio of gas to dust, humidity of the flow, temperature of the flow, particle average size, gas and/or particle chemical properties and the ever present need to conserve energy, reduce operational expense, and require minimum space for the filtration facility.
Regardless of the specific filtration technique that is used, the separated particulate matter or dust periodically must be physically removed from the facility, and that dust which clings closely to the fabric must be substantially separated away therefrom. One commonly used technique is to briefly but deliberately reverse the air flow through the fabric, thereby dislodging a substantial amount of the dust which otherwise would tend to impregnate and clog the spaces between the fabric fibers. Such systems are often called "reverse air" systems.
It is well known that similarly charged particles repel each other. This phenomenon can be exploited in electrostatically enhanced fabric filtration systems by providing the dust particles with a charge of a particular polarity, such that they will not clog up the filter element because they tend to repel each other adjacent to that filter element. As a result, a charged gas-permeable cake of similarly charged particles locates adjacent the air filter and continues to repel other particles while permitting the carrier gas to flow through the particle assembly and the filter itself.
U.S. Pat. No. 3,910,779 to Penney, titled "Electrostatic Dust Filter", discloses such a system in which the particulates in a gas stream are electrically charged in a corona region and are then carried by the gas stream to a separate filtering region downstream of the corona charging region. In the filtering region, a textile fabric is mounted on a metallic support structure, with a non-corona electric field being maintained at the collecting surfaces of the filter. According to this technique, no corona should be permitted near the particulate collecting surfaces of the filter.
U.S. Pat. No. 4,354,858 to Kumar et al, titled "Method for Filtering Particulates", discloses a filter medium comprising a porous cake, composed of electrically charged particulates previously drawn from the gas stream and collected on the upstream one of two foraminous support structures. The apertures of this upstream foraminous support structure are larger than the average size of the particulates that are to be filtered from the gas stream by more than an order of magnitude. The two foraminous structures are placed adjacent to each other and both are grounded at a point downstream of a plurality of high voltage corona discharge electrodes. As the charged particles pass through the upstream one of the pair of grounded foraminous structures, some of them eventually attach themselves to the downstream structure and build up a charge which repels like charged particles that are approaching it with the carrier gas. Shortly, a porous particulate cake builds up upstream of the upstream foraminous structure and permits the carrier gas to pass through. Kumar et al disclose one embodiment that has the form of two coaxial cylinders, of which the inner one has the larger apertures and constitutes the upstream foraminous structure. Gas flows axially into the cylindrical structure and radially outward through both the foraminous structures which are grounded. An axially oriented central electrode coacts with the foraminous structures to provide a radial electrostatic field.
The Penney structure with its corona generating field and a physically separated array of unidirectional electric fields between neighboring filter elements is relatively complex and requires a substantial amount of space. The Kumar et al system requires two carefully graduated foraminous structures, both of which are electrically conducting and grounded.
A need therefore exists for an electrostatically enhanced fabric filtration system to separate particulate matter or dust from a gas stream, which has a relatively simple structure, utilizes known filter fabrics, is inexpensive to build and maintain, requires little space, and is operable with relatively simple electrical circuitry.